Some 465 patients with clinically isolated syndrome (their first neurological episode, caused by inflammation or demyelination of nerve tissue) were randomized to early vs late treatment with interferon beta-1b (IFNB-1b). Compared with those who had 25-hydroxyvitamin D serum concentrations less than 50 nmol/L in the first 12 months following their first neurological episode, those with serum levels of 50 nmol/L or greater had a significantly lower rate of new active lesions and relapse, a significantly lower yearly increase in T2 lesion volume, and lower disability during the following 4 years.